1. Field of the Invention
The present invention relates to a method for operating an implantable cardiac stimulator of the type having atrial overdrive capability, and having an atrial stimulator for stimulating the atrium via stimulation electrode(s), an atrial evoked response detector for determining an atrial evoked response amplitude, and an atrial control unit that controls an atrial timer to set an atrial stimulation time interval length between consecutive atrial stimulation pulses.
2. Description of the Prior Art
A healthy heart pumps blood through the circulatory system in successive, periodic cycles each including an atrial contraction followed shortly thereafter by a ventricular contraction. The successive atrial and ventricular contractions occur upon being triggered by the heart's natural pacemaker, which causes electrical wave fronts to propagate through cardiac tissue, causing the tissue cells to be momentarily depolarized, thereby initiating the contractions. If a patient's natural pacemaker, through disease, ceases to function or functions only erratically, artificial pacing therapy can be provided by an implanted pacemaker, which delivers low-energy pacing pulses to the atrium, or to the ventricle, or to both the atrium and the ventricle in a properly synchronized sequence. Depending on the needs of a particular patient, the pacemaker can be programmed to continuously supply such pacing pulses without interruption, or can operate to sense when the patient's natural pacemaker has failed to deliver a signal resulting in contraction, and only then does the implanted pacemaker deliver a pacing pulse. Pacemakers of this latter type are known as demand pacemakers.
Fibrillation, in general, characterizes abnormal operation of the heart, which can spontaneously occur, wherein the normal initiation of the electrical wave fronts becomes chaotic and therefore the cardiac tissue never receives a clear or coherent signal triggering contraction, and pumping therefore ceases. Ventricular fibrillation is a life-threatening condition, and when it occurs must be treated rapidly and effectively. For this purpose, implantable defibrillators are well-known in the art, which deliver one or more high-energy electrical pulses to the cardiac tissue, at selected locations and in a selected timing sequence, so as to momentarily depolarize substantially all of the cardiac tissue, thereby rendering virtually all of the cardiac tissue momentarily unable to propagate the chaotic wave fronts. If defibrillation is successful, when the cells again become capable of propagating a pacing wave front, they will do so in a normal, non-chaotic manner.
Atrial fibrillation is usually not an immediately life-threatening pathology, and can be tolerated for a certain amount of time without significant adverse consequences to the patient. This means that upon the occurrence of atrial fibrillation, there is usually a relatively long time during which an effective therapy can be developed, and subsequently administered. Although implantable defibrillator technology, primarily intended for treating ventricular fibrillation, can be adapted also to treat atrial fibrillation, the delivery of high energy shocks to the patient is painful and moreover, such drastic therapy is usually not necessary in the case of atrial fibrillation. Atrial fibrillation is also treated by extracorporeal delivery of the shocks to the heart through the skin of the patient by an external defibrillator of the type well-known in the art, also being extremely uncomfortable for the patient. Moreover, this type of treatment generally results only in temporary relief for patients, and must be repeated.
In treating atrial fibrillation by means of electrical shocks supplied to the heart, such shocks must be applied in synchronism with the ventricular electrical activity, otherwise ventricular fibrillation may be induced.
Another treatment regimen for atrial fibrillation is the administration of suitable drugs for reducing the occurrences of atrial fibrillation. Drugs suitable for this purpose which are currently available, however, have many undesirable side effects, and many patients become resistant to their atrial fibrillation suppressing properties, thereby significantly reducing the therapeutic effect of such drugs.
Recent pacemakers include an overdrive capability adapted to control the mechanisms responsible for atrial fibrillation (AF). Herein, overdrive is defined as a pacing regime that suppresses the initiation of atrial fibrillation by stimulating the atrium at a rate higher than the patient's own intrinsic atrial rate. St Jude Medical has designed an AF suppression algorithm, known as the Dynamic Atrial Overdrive (DAO) algorithm for that purpose, see e.g. the brochure “AF Suppression Algorithm: A New Tool for Reducing Atrial Fibrillation in Pacemaker Recipients” © 2001 St Jude Medical Cardiac Rhythm Management Division. It accomplishes this by continually monitoring the intrinsic atrial rhythm, promptly increasing the stimulation rate when the intrinsic atrial rhythm emerges, and periodically reducing the stimulation rate gradually to search for intrinsic atrial activity. This process ensures that the stimulation rate is not inappropriately rapid when the patient is at rest, yet is sufficiently high when the patient is active.
With AF suppression turned on, detection of two intrinsic atrial events within a 16-cycle window causes an increase in the atrial stimulation rate. The magnitude of the increase depends among others on the current stimulation rate. The increased rate is maintained for a programmed number of overdrive cycles, after which the system begins to search for the intrinsic rate by gradually extending the atrial stimulation interval (8 ms for rates >100 ppm; 12 ms for rates <100 ppm).
The described algorithm is illustrated by the ECG shown in FIG. 1. In FIG. 1, while overdrive pacing the atrium at a rate of 84 ppm, the device detects two intrinsic atrial events and immediately responds with a rate increase (by overdrive pacing at a rate of 93 ppm). After stimulating the atrium for the selected number of overdrive pacing cycles (15 cycles at a rate of 93 ppm), the device begins to extend the pacing cycle lengths to search for intrinsic atrial activity (rate recovery). Detection of two intrinsic atrial events would again initiate a prompt rate increase and reset the cycle length counter.
In the presently used AF suppression algorithm, which briefly has been described above, the paced atrial rate is decreased until two spontaneous P-waves are seen within a 16-cycle window. The algorithm then increases the paced atrial rate to regain a paced atrial rhythm, where after it again starts to decrease its pace rate until two spontaneous P-waves are seen within a 16-cycle window.